The control of RF signals in a frequency range of up to 500 megahertz has been accomplished employing the previous invention of one of the inventors of this device and is disclosed in U.S. Pat. No. 3,550,041. The signal controllers disclosed in that patent were designed to accomplish the same general results as this invention and proved eminently satisfactory to provide isolation as great as 80 db with power levels up to -20dbm at frequencies up to 500 megahertz. That system involved a plurality of semiconductive devices such as PIN diodes arranged in series with the signal path with independent bias establishing means for controlling the bias and thus the transmission through the signal controller.
We have found that the effects of frequency on the PIN diode impedance and its thermal energy absorption limitations tend to limit the instantaneous bandwidth frequency response signal and power handling capability of this type of controller.
Other prior art known to the applicants are the U.S. Pat. No. 3,559,108 to H. Seidal showing use of PIN diodes in combination with hybrid couplers and switches with selectable bias supplies to switch PIN diodes into conducting, non-conducting condition; and U.S. Pat. No. 3,673,492 to R. A. Gilson showing the use of PIN diodes in combination with hybrid networks to attenuate energy as a function of a control voltage. The PIN diodes are connected in parallel with a terminating resistance.
The coupler switches of the U.S. Pat. No. 3,559,108 do not dissipate energy in the diodes but merely act as switches and reflect energy back into the input line when in an open state. This reflected energy can be re-reflected back into the diodes by discontinuities or impedance mismatch in the input line or the source, to distort and degrade any signals conducted through the switch. Switches of such type are unsatisfactory for high resolution signal controller applications when the energy that is not intentionally reflected to the output port is not dissipated.
The attenuator of U.S. Pat. No. 3,673,492 employs characteristic impedance terminating resistors which limit the maximum impedance seen at the quadrature terminals of a quadrature device. This prevents the impedance from ever exceeding that of the characteristic impedance resistors and no polarity reversing can be accomplished.